TERRAIN ANALYSIS USING LIDAR TOPOGRAPHIC DATA: A CASE HISTORY FROM WILLISTON, NORTHWEST VERMONT

Surficial geologic mapping for the Vermont Geological Survey was greatly facilitated by the use of LIDAR (LIght Detection And Ranging), a form of airborne laser scanning. Williston, on the eastern side of the Champlain Valley, was an ideal test site due to the varied topography.

Lidar data obtained from the Vermont Mapping Program had already been processed to remove the effects of vegetation and buildings and was available as a grid of elevation points with 3.2 m spacing. Using ArcGIS™ software, an 8 m raster digital elevation model (DEM) was created and contour lines, percent slope DEMs, and shaded relief (SR) DEMs were derived from it. Tests of slope maps derived from 4, 8, 16, and 32 m DEMs showed that the 16 and 32 m DEMs were inadequate for 1:24,000 mapping due to lack of detail.

The slope map dramatically reveals the fine texture of the landscape, with subtle changes in slope being shown through a continuous shading from light to dark. By this means, many features having less than one meter of topographic relief are clearly shown. This allows field mappers to rapidly discern meander scrolls in the floodplains, lacustrine and fluvial terraces, gullies eroded into the terraces, till shadows, hummocky till, and many bedrock outcrops, whether in open or forested areas. Bedrock-controlled lineaments show up clearly. Wave-washed till slopes within the Upper and Lower Fort Ann stages of glacial Lake Vermont reveal subtle, contour-parallel “bathtub rings” that appear to represent down-stepping shorelines. The map is also effective for discerning contacts between surficial deposits, such as between a lacustrine terrace and a till slope.

Although SR maps are also effective, multiple illumination directions are commonly needed because the maps accentuate slopes facing away from the illumination source at the expense of illuminated slopes. SR maps were produced with illumination from azimuths 045°, 135°, 225°, and 315°. No one of these could show all of the features revealed by the slope map.

The slope map can portray much of the subtle terrain texture that was formerly only available from stereoscopic interpretation of aerial photos. Combined with digital orthophotos and high-resolution GIS surface water and road layers, it is a highly effective tool for mapping surficial deposits and landforms.